Big solar plants, which are referred to as solar power plants, consist of a plurality of solar modules or of photovoltaic modules and inverters. The photovoltaic modules acting as the generators are connected in series or in parallel to form what are referred to as strings. The inverters are wired so that they feed the grid via a joint mains transformer.
In principle, one-phase or multi-phase inverters can be utilized. Three-phase inverters are primarily used.
It is known to dispose one-phase inverters in such a manner that feeding occurs in any phase of a multi-phase mains transformer. For each phase, there is at least one inverter. The inverters are connected to the phases or to the conductors L1, L2, L3 through switches, in particular through contactors and, as a result thereof, to the primary side of the transformer. On the primary side, the transformer windings are star-connected. The midpoint is connected to the neutral conductor “N.” The neutral conductor “N” is connected to ground (PE). The windings of the secondary side preferably have a triangular connection.
If a multi-phase inverter, more specifically a three-phase inverter, is used, the outputs of the inverter are connected to the conductors L1, L2, L3 via contactors.
Some photovoltaic modules, namely thin-film modules, must be operated in such a manner that a certain orientation of the field strength cannot occur at their cells. Otherwise, early damage may occur to these modules, resulting in a shorter life.
A known solution is to ground a terminal of the photovoltaic generator. As a result, the cells of the modules can only accept voltages having a polarity to ground.
The grounding of the modules however entails the use of either galvanically separating inverters or of transformerless inverters of certain topologies. Both inverter types must internally generate a missing polarity of the input voltage in order to be capable of feeding into the grid. This leads to a reduced conversion efficiency of these inverters.
By contrast, another solution is known from the document DE 20 2006 008 936 U1. In this solution, a certain potential is applied to a module terminal. A device is provided through which the negative connection of a module is raised to a positive potential. This is intended to lengthen the life of the modules. The advantage of this device is that inverters of the most various topologies can be utilized.
If the solar plant is big and has many photovoltaic generators and inverters, a large number of such devices should be mounted. This approach, however, is expensive.